This invention generally relates to semiconductor light emitting devices, and more particularly to a novel structure for Gallium Nitride (GaN)-based semiconductor devices.
It has been demonstrated in the art that multiple quantum well (MQW) structures can for optical lattices in which different quantum wells are coherently coupled due to interaction with a retarded electromagnetic field. Light-matter interaction in MQW structures depends on their structure and can be significantly and controllably modified. The III-V nitrides for use with MQW structures have long held promise for optoelectronic applications in the blue and ultraviolet wavelengths and as high-power, high-temperature semiconductors, but only recently have practical devices been developed.
FIG. 1 is a block diagram that schematically illustrates the structure of a Gallium Nitride (GaN)-based III-V compound semiconductor light emitting diode (LED) in the art. The structure 100 has a substrate 101 having an n-GaN layer 102 disposed co-extensively thereupon. An n-type semiconductor is a semiconductor type in which the density of holes in the valence band is exceeded by the density of electrons in the conduction band. N-type behavior is induced by the addition of donor impurities, such as silicon, germanium, selenium, sulfur or tellurium, to the crystal structure of III-V nitrides. A multiple quantum well (MQW) layer 103 is disposed on n-GaN layer 102 flush on one side of n-GaN layer 102 and an n-electrode 104 is disposed non-extensively opposite MQW layer 103 on the surface of n-GaN layer 102. A p-GaN layer 105 is deposited on MQW layer 103 flush therewith and a transparent conductive layer 106 is deposited flush on p-GaN layer 105. A p-type semiconductor is a semiconductor type in which the density of electrons in the conduction band is exceeded by the density of holes in the valence band. P-type behavior is induced by the addition of acceptor impurities, such as beryllium, strontium, barium, zinc or magnesium, to the crystal structure of III-V nitrides. A p-electrode 107 is disposed non-extensively upon transparent conductive layer 106.
FIG. 1A is a block diagram that schematically illustrates the structure of a Gallium Nitride (GaN)-based III-V compound semiconductor laser diode (LD) in the art. The structure 100a includes a substrate 101a having an n-GaN layer 102a disposed co-extensively thereon. An n-cladding layer 108a is disposed on n-GaN layer 102a flush on one side thereof and an n-electrode 104a is disposed non-extensively opposite the n-cladding layer 108a on the surface of the n-GaN layer 102a. A multiple quantum well (MQW) layer is disposed co-extensively on the n-cladding layer 108a. Moreover, a p-cladding layer 106a is disposed co-extensively on the MQW layer 103a. A p-GaN layer 105a is disposed co-extensively on the p-cladding layer 106a. A p-electrode 107a is disposed co-extensively on the p-GaN layer 106a. 
In such conventional structures as described in conjunction with FIGS. 1 and 1A, because the p-type III-V nitrides are grown after the MQW, and require a relatively high growing temperature, then in order not to influence the structure and quality of the MQW, the p-GaN growing temperature should not be too high and the growing time should not be too long. In this case, the p-GaN hole concentration, crystal quality, and thickness cannot be improved. Additionally, in LEDs, because p-GaN can absorb light emitted from the MQW, then if the thickness of the p-GaN layer increases, it will adversely influence the effectiveness of light emission. However, if the hole concentration of the p-GaN layer cannot be increased, it will make its sheet resistance extremely high, so when current flows through it, it will tend towards vertical conduction and not the desired horizontal diffusion (current spreading) on the element""s surface. When the p-GaN film thickness decreases, this phenomenon will be clearly evident, significantly decreasing the LED""s light emitting effectiveness and the light emitting region""s size.
Prior art solutions include depositing a thin, transparent metal conducting layer on the p-GaN surface in LEDs and using this conductive layer to make the current spread evenly over the element""s surface, thereby increasing the light emitting region and its effectiveness. However, because p-GaN has an extremely high work function, no metal can act in conjunction with it to effectively form a natural ohmic contact. Effective ohmic contact is crucial since the performance of semiconductor devices such as operating voltage is strongly influenced by the contact resistance. Moreover, it is difficult to increase the concentration of p-GaN, and the p-GaN surface is easily contaminated by airborne particles and oxidized. These factors make it difficult to achieve an effective ohmic contact between the p-GaN and a metal conducting layer thereby influencing the electrical properties. There have been other attempts in the art to solve these problems, including utilizing different types of metal layers, surface decontamination, and background gases and heat treatment, but they have all failed to provide a satisfactory ohmic contact. In addition, the transparency between the metal conductive layer and the p-GaN contact in the art cannot reach 100 percent. These and other shortcomings in the art have created a general need for an optimal semiconductor light emitting device structure, and more particularly, a novel and optimal structure for Gallium Ar Nitride (GaN)-based III-V compound semiconductor light emitting devices including LEDs and LDs.
According to a preferred embodiment of the present invention, there is provided a novel and optimal semiconductor light emitting device comprising a substrate, an n layer disposed co-extensively on the substrate, an n++ layer disposed non-extensively and flush on one side of the n layer. Furthermore, a p+ layer is disposed co-extensively on the n+ layer of the device according to the invention, with a p layer further disposed co-extensively on the p+ layer. A p cladding layer is disposed co-extensively on the p layer. A multiple quantum well (MQW) layer is disposed co-extensively on the p cladding layer, and an n cladding layer is further disposed co-extensively on the MQW layer. A second n layer is disposed co-extensively on the n cladding layer. An n+ layer is disposed co-extensively on the second n layer of the device according to the invention. After partially etching the device, an n electrode is etched opposite n++ layer non-extensively on the surface of n layer, and a second n electrode is formed non-extensively (without etching) upon the n+ layer.
The invention provides a corresponding method for manufacturing a semiconductor light emitting device. This preferred embodiment of the method according to the invention comprises the steps of forming an n layer co-extensively on a substrate, forming an n++ layer non-extensively and flush on one side of the n layer, forming a p+ layer co-extensively on the n++ layer, forming a p layer co-extensively on the p+ layer, forming a p cladding layer co-extensively on the p layer, forming a multiple quantum well (MQW) layer co-extensively on the p cladding layer, forming an n cladding layer co-extensively on the MQW layer, forming a second n layer co-extensively on the n cladding layer, forming an n+ layer co-extensively on the second n layer, partially etching the light emitting device, forming an n-electrode opposite said n++ layer and non-extensively on the n layer, and forming a second n-electrode non-extensively on said n+ layer.
The invention further provides an additional embodiment of a light-emitting diode (LED) according to the invention. The LED according to this particular embodiment includes a sapphire substrate having an n-type GaN layer disposed co-extensively thereupon. An n+ GaN layer is disposed non-extensively and flush with one side of n-type GaN layer. An ohmic contact is formed opposite the n+ GaN layer non-extensively on the surface of the n-GaN layer. A p-type GaN layer is disposed co-extensively with the n+ GaN layer. A multiple quantum well (MQW) layer, made of indium gallium nitride (InGaN) and gallium nitride (GaN), is disposed co-extensively with p-type GaN layer and formed thereupon. Another n-type GaN layer is disposed co-extensively with the MQW layer and formed thereupon. A second ohmic contact is formed non-extensively upon the n-type GaN layer. The ohmic contacts can be formed from titanium (Ti), aluminum (Al) or gold (Au) in a number of multi-layered combinations.
The invention further provides a further embodiment for forming a light-emitting diode (LED) according to the invention. The method according to this particular embodiment of the invention comprises the steps of forming an n-type GaN layer co-extensively on a sapphire substrate, forming an n+ GaN layer non-extensively and flush on one side of the n-type GaN layer, forming a p-type GaN layer co-extensively on the n+ GaN layer, forming a multiple quantum well (MQW) layer (made of InGaN or GaN) co-extensively on the p-type GaN layer, forming a second n-type GaN layer co-extensively on the MQW layer, partially etching the LED, forming an ohmic contact opposite the n+ GaN layer non-extensively on the surface of the n-GaN layer, and forming a second ohmic contact non-extensively on the n-type GaN layer. The ohmic contacts can be formed from titanium (Ti), aluminum (Al) or gold (Au) in a number of multi-layered combinations.